The processing of rubbers into shaped objects is normally a multistage procedure. In contradistinction to the manufacture of molded components from thermoplastic synthetic resins, it is normally necessary, prior to the final shaping step, to conduct generally complicated mixing procedures to obtain a homogeneous incorporation of diverse additives, such as cross-linking agents. The relatively high viscosity of these raw materials makes it necessary to utilize heavy machinery such as rolling mills or internal mixers, which consume large amounts of energy. The inherent costs of this energy and equipment are considerable and greatly impair the economy of the production of elastomeric materials.
The statistical distribution homogeneity required of the various components in the elastomer mixtures requires a considerable expenditure of time and energy in addition to high investment costs. During the solid state homogenization, the rubber particles and filler particles are displaced with respect to one another against a strong resistance. Even those portions of the mixture which already have been sufficiently intermixed must still participate in the mixing procedure until a satisfactory dispersion of the entire mixture has been accomplished. The frictional heat generated requires a discontinuous mode of operation. In order to avoid or minimize the danger of premature vulcanization, the vulcanizing agents or a portion thereof are not added until a second operating step. However, the preparation of cross-linkable elastomers sufficiently stable for even intermediate storage periods requires expensive measures and is effected, in modern large-scale plants, by expensive pelletizing and storage in rotating containers. Further processing, involves the subsequent preparation of a blank on calenders and/or extruders, depending on the type of the finished article. The subsequent conventional vulcanization in presses or autoclaves completes the manufacturing process.
In order to avoid large energy expenditures, to eliminate the need for expensive mixing equipment, such as Banbury mixers, and to allow for the use of automated equipment in the handling of rubbers during processing pulverulent elastomeric mixtures have been developed. In fact, a large number of published references deal with such powdered rubbers. However, even though they offer many advantages powdered rubbers are not widely used today and only recently have methods been disclosed for making pulverulent pourable elastomer mixtures, such as rubber/carbon black mixtures.
U.S. Pat. No. 3,920,604 and U.S. Pat. No. 3,945,978 describe processes of preparing an elastomer containing a filler by emulsifying a solvent solution of the elastomer and the filler, flashing off the solvent and coagulating the mixture to a pourable powder.
Also of interest is U.S. Pat. No. 4,073,755, which describes a process for the production of pulverulent, tack-free, pourable filler-containing elastomer particles by precipitating a stable emulsified homogeneous mixture of an aqueous elastomer emulsion and an aqueous dispersion of a solid filler in the presence of sodium silicate, separating the thus precipitated elastomeric mixture from water and drying the mixture to a tack-free powder. Although this method initially gives a tack-free powder, the powder tends to conglomerate on storage when exposed to the atmosphere. This is probably caused by the silicic acid used to coagulate the mixture. After coagulation the resultant acid solution is neutralized to form a salt which is partly incorporated in the elastomer-filled powder and is not readily removed even by washing. When exposed to the atmosphere the resultant salt deliquesces causing the particles to conglomerate.
A method making a resin encapsulated elastomer is described in U.S. Pat. No. 3,813,259. In that invention the rubber is coated with a resin partitioning agent by agglomerating the resin from its latex at a specific temperature which must be within 5.degree. C. of the agglomeration temperature of the resin. This temperature is critical for the adherence of the resin coating to the elastomer.
U.S. Pat. No. 4,375,497 discloses a process for the production of tack-free, pourable, filler containing elastomer powder which comprises;
(a) dispersing a carbon black filler in water; PA1 (b) mixing the thus dispersed carbon black filler with an elastomer latex; PA1 (c) coagulating the thus produced mixture of elastomer latex and carbon black with an acid/alum solution: PA1 (d) partitioning the thus coagulated suspension with a coating resin: and PA1 (e) filtering, washing and drying of the resultant powder, wherein the coating resin is comprised of a styrene/butadiene resin and a styrene/.alpha.-methylstyrene resin or a polystyrene resin. PA1 (a) dispersing a carbon black filler in water; PA1 (b) mixing the thus dispersed carbon black filler with an elastomer solution and a surfactant to produce an elastomer emulsion; PA1 (c) coagulating the elastomer emulsion; PA1 (d) partitioning the coagulated elastomer emulsion with a coating resin which is comprised of at least one copolymer containing from 70% to 97% by weight vinyl aromatic monomers and from 3% to 30% by weight diene monomers: and PA1 (e) filtering, washing and drying the resultant powder.